1. Field of the Invention
The present invention relates to an ink jet recording method, in particular, to an on-demand type ink jet recording method.
2. Related Background Art
Assuming that the size which substantially decides the resolution of a recording object, that is, the minimum unit of a recorded image, is called a recording unit, the recording unit of almost all of the ink jet recording methods which have conventionally been used is a dot. One dot denotes an image obtained as a result of one liquid droplet that was deposited on a recording medium such as a recording paper or the like. That is, the discharging density of liquid droplet determines the resolution after recording, namely, the recording unit.
The above point will be further described with respect to an example of a dither method as one of the image expressing methods. In the dither method, a concept of a pixel is used. One pixel comprises a plurality of dots and the gradation is expressed by changing the number of dots. That is, the dots decide the resolution of a recording object, i.e., the recording unit. Therefore, the recording unit in the dither method is determined by the dot density.
On the other hand, hitherto, a coated paper has generally been used as an ink jet recording paper. However, there is a large demand to use ordinary paper in consideration of the running costs, using efficiency, and the like. The paper which will be used most commonly in the future will certainly change from the coated paper to the ordinary paper.
There are many technical problems upon changing from the coated paper to the ordinary paper. Among them, when the conventional technique is applied to the ordinary paper, bleeding of the ink becomes anisotropic, so that a dot shape deteriorates and quality deterioration occurs. In addition, what is called a fixing time, which is the time required until a hand does not become dirty even when the hand touches the ink after completion of the printing, becomes long. The above two points are the significant subjects which are inevitable and could hinder widespread use of ordinary papers. FIGS. 1A and 1B are diagrams for explaining the fixing time.
FIG. 1A shows the case where a liquid droplet having a diameter of d was deposited onto a surface X-X' of a coated paper and became a dot of a diameter of .alpha..sub.a d. FIG. 1B shows the case where a liquid droplet of the diameter of d was deposited onto the surface X-X' of the ordinary paper and became a dot of a diameter of .alpha..sub.b d and .alpha..sub.b denote ratios (hereinafter, referred to as bleeding ratios) of the dot diameter to the liquid droplet diameter. In general, the bleeding ratio of the coated paper is larger than that of the ordinary paper and there is a relation of .alpha..sub.a &gt;.alpha..sub.b. Therefore, an amount of ink which should be received per unit area of the ordinary paper is larger than that of the coated paper. In FIGS. 1A and 1B, consideration has been made with respect to a model such that the liquid droplets having the same diameter d are transformed to the circular cylinders each having the same bottom area as that of the dot and are fixed onto the papers. In this case, heights H.sub.a and H.sub.b of the circular cylinders are proportional to the inks to be received per unit area of the papers. There is a relation of H.sub.a &lt;H.sub.b between the heights H.sub.a and H.sub.b of the circular cylinders on the coated paper and the ordinary paper as will be understood from FIGS. 1A and 1B. Since the fixing time increases as the circular cylinder is high, it takes a longer time to fix the ink onto the ordinary paper and such a long time causes a serious obstacle in high printing speed.
The dot shape will now be described. Unlike the coated paper, the ordinary paper does not have a special ink absorbing layer. Therefore, since the ink must be fixed into the space in which fibers and a sizing material such as a resin or the like are complicatedly mixed, the bleeding of the ink is anisotropic and the dot shape becomes complicated. Further, the ink runs along the fibers by capillarity and results in a dot with a mustache-like protrusion. In many cases, a serious adverse influence is exerted on the printing quality. Such a problem will now be considered with respect to a cylindrical model. Since an amount of ink to be received per unit area of the paper increases as the height of the circular cylinder rises, the dot shape also deteriorates. That is, for the ordinary paper, there is a tendency to deteriorate the dot shape from not only the quality of the paper but also the cylindrical model.
As mentioned above, there is a drawback such that when the recording is executed on ordinary paper by an ink jet recording method which has conventionally been used for the coated paper, both of the fixing and printing qualities deteriorate. Therefore, various methods have been proposed to solve such a drawback. For instance, according to JP-A-56-57862, JP-A-57-102970, and JP-A-57-102971, the ink of a strong base of about pH 13 is used, the sizing material or the like on the paper surface is dissolved and penetrated, thereby forming a circular dot having a good fixing performance. However, such a method has a problem of safety when dealing with the strong base. Although the ink can be preferably fixed and printed onto an acid paper using the resin or the like as a sizing material, for neutral papers in which a production amount has been increasing more and more in recent years, there is a tendency such that the fixing performance remarkably deteriorates and the printing quality also slightly deteriorates. In addition, the ink of the strong base also has a drawback such that since the permeable force of the ink of the strong base into the paper is large, the ink easily reaches the back side and it is extremely difficult to record on both sides of the paper.